The version of (REF -REF ) in [1]} sets the dead zone variable as \( D=0 \) . In face of persistent structural perturbations in \( e_{ik} \) , \( l_2=l_1 \)  cannot be sustained. Consequently, regardless of the size of perturbations, with \( D=0 \) , \( \hat{x}_i \)  will regularly rise to the limit of the modulation threshold \( M \) , then fall, and then rise again. On the other hand we will show that if \( D \)  is sufficiently greater than \( \epsilon \) , the bound on the perturbation, then (REF -REF ) will have ultimate bounds proportional to \( \epsilon . \)  This raises an essential trade-off. Too large a \( D \)  slows convergence though imparts greater robustness to perturbations. Such a compromise is inherent to most dynamic systems. Slower convergence improves noise performance.
